btn3.onclick = function chouka() {

    var chu = Math.floor(Math.random() * 10);
    if (0 <= chu && chu < 7) {
        location.href = 'lan.html';
    } else if (7 <= chu && chu < 9) {
        location.href = 'zi.html';
    } else {
        location.href = 'jin.html';
    }
}